Method for determining nitrogen oxide content in internal combustion engine exhaust gases containing oxygen

ABSTRACT

An operating internal combustion engine with at least one cylinder ( 2 ), and a piston ( 12 ), which can move in an alternating manner therein, is used to compress a fuel mix in a combustion chamber ( 11 ). In order to determine the nitrogen oxide content in oxygen-containing exhaust gases, the quantity of fuel fed to the cylinder ( 2 ) and the air mass flowing in an induction pipe ( 15 ) are recorded and are fed to an electronic circuit ( 6 ). The center of gravity (S) of the combustion is determined from at least one current measured value for the engine operation, and the level of nitrogen oxide emissions is calculated from the value for the center of gravity (S) of the combustion, including the values for the recorded fuel quantity and air mass.

[0001] The invention relates to a method for determining the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines.

[0002] When internal combustion engines are operating, exhaust gases which contain various pollutants are formed, the levels of these pollutants being dependent substantially on the composition of the fuel/air mix. Particularly in the case of operation with a lean fuel/air mix, i.e. lambda>1, the level of nitrogen oxides (NO_(x)) is high. To ensure that the exhaust emissions regulations, which are in some countries highly stringent, can be observed, it is known to use NO_(x) storage catalytic converters. However, despite being regenerated during certain operating conditions, these NO_(x) storage catalytic converters have only a limited storage capacity, and consequently it is not always possible to store sufficient amounts of the nitrogen oxides produced.

[0003] To combat this problem, DE 198 01 626 A1 has already proposed a method for diagnosis of a catalytic converter in the exhaust gas from internal combustion engines which has a capacity to store both oxygen and nitrogen oxides. In this method, it is provided that a first phase shift between a lowering of the oxygen concentration and a subsequent reaction of the sensor and a second phase shift between a subsequent increase in the oxygen concentration and a following reaction of the sensor are recorded. In this method, the difference in the phase shift is determined and a fault signal is stored and/or emitted if the said difference does not reach a predetermined threshold. With this method, it is not possible to influence the operation of the internal combustion engine and the level of nitrogen oxides in the exhaust gas formed during combustion.

[0004] EP 0 783 918 A1 has disclosed a method for lowering the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines, in particular from diesel engines and direct-injection spark-ignition engines for motor vehicles. In this method, the nitrogen oxides are reduced at a catalytic converter with the aid of a reducing agent which is metered to the exhaust gas as a function of operating parameters. The reducing agent used is hydrogen and/or hydrocarbon, with only hydrogen being fed to the exhaust gas upstream of the catalytic converter in a first operating mode of the internal combustion engine, with both hydrogen and hydrocarbon being fed to the exhaust gas upstream of the catalytic converter in a second operating mode and only hydrocarbon being fed to the exhaust gas upstream of the catalytic converter in a third operating mode. In this case too, it is not possible to influence the way in which the internal combustion engine operates with regard to the formation of the nitrogen oxide fraction.

[0005] The invention is therefore based on the object of providing a method for determining the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines, by means of which it is possible to determine the nitrogen oxide emissions on the basis of the variables which actually have an influence.

[0006] This object is achieved by a method for determining the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines having the features of claim 1.

[0007] In the development of internal combustion engines with fuel injection, it has already been attempted for some time to determine the nitrogen oxide emissions (NO_(x) emissions) by calculation. Achieving this determination would help, for example, to precalculate the NO_(x) emissions and with test planning and also with plausibility checks of measured values, such as indexing data and NO_(x) values. However, the current simulation models which are used to determine the NO_(x) emissions by calculation are altogether inadequate. Moreover, on account of the extremely high demand for calculation time, these calculation models are unable to form a control algorithm for use in vehicles.

[0008] This problem is also of particular importance in connection with the use of SCR catalytic converters. The quantity of urea to be injected for a catalytic converter of this type is in a fixed ratio to the NO_(x) emissions. From this, it can be concluded that correspondingly accurate metering of the urea is possible as a function of the accuracy with which the NO_(x) emissions can be determined, and therefore the efficiency of the catalytic converter can be increased.

[0009] The present invention makes it possible to precisely calculate the NO_(x) emissions, since this calculation is based on values from the variables which actually have an influence on the NO_(x) emissions. The level of the NO_(x) emissions from an internal combustion engine is dependent primarily on the local temperature, the oxygen concentration and the residence time of the cylinder charge in the combustion chamber. The two latter variables can be recorded relatively easily by measuring the engine speed of the air used and also the fuel quantity. On other hand, it is much more difficult to determine the gas temperature in the combustion chamber. The present invention therefore proposes using a different variable which is directly linked to the gas temperature which is of relevance to the formation of nitrogen oxides. Since the gas temperature is decisively dependent on the center of gravity of the combustion, i.e. the position where 50% of the fuel is converted in relation to the piston position TDC, it is advantageous to select the center of gravity or a similar variable, such as for example the position of the maximum energy conversion, as a reference variable for the NO_(x) emissions. The level of the NO_(x) emissions is calculated from this value for the center of gravity of the combustion and the values of the recorded fuel quantity and air mass, for example with the aid of neural networks.

[0010] The determination of the center of gravity of the combustion is preferably effected by measuring the combustion-chamber pressure profile. For this purpose, a pressure sensor is provided in the region of the combustion chamber. This way of determining the center of gravity of the combustion is extremely precise. Alternatively, it is also possible to use a dedicated model for calculating the center of gravity from the start of injection to determine the center of gravity of the combustion.

[0011] If there are pressure sensors for determining the center of gravity of the combustion, there are also further advantages, in particular with regard to the monitoring of the maximum pressure for fault detection, for establishing the operating mode and the like.

[0012] In a further configuration of the invention, it is advantageous if the quantity of recirculated exhaust gas is recorded by means of a sensor and a corresponding signal is fed to the electric circuit, and this signal is included in the calculation of the level of the NO_(x) emissions. Furthermore, it is advantageous if the oxygen concentration in the exhaust gas is recorded and a corresponding signal is fed to the electric circuit and if this signal is included in the calculation of the level of the NO_(x) emissions. To monitor all the cylinders and to carry out a comparison of the corresponding pressure profiles for the purpose of fault detection, it is advantageous for a pressure sensor to be arranged in each cylinder, so that the pressure profile in the combustion chamber is recorded in each cylinder, and a separate calculation of the NO_(x) emissions takes place for each cylinder.

[0013] Furthermore, in the case of fast-running internal combustion engines, it is expedient for the rotational speed of the internal combustion engine to be recorded and for a corresponding signal to be fed to the electric circuit, and for this signal to be included in the calculation of the level of the NO_(x) emissions. Moreover, it is expedient to provide an NO_(x) sensor which records the NO_(x) content in the exhaust-gas stream, the resulting measured value being compared with the level of the calculated NO_(x) emissions.

[0014] The invention is explained in more detail below with reference to the drawing, in which:

[0015]FIG. 1 diagrammatically depicts an engine block with pressure sensors and engine electronics,

[0016]FIG. 2 diagrammatically depicts a vertical section through an internal combustion engine with fuel and air feed,

[0017]FIG. 3 illustrates the profile of the combustion and position of the center of gravity, based on the crank angle,

[0018]FIG. 4 illustrates the way in which the nitrogen oxide emissions are dependent on the position of the center of gravity, based on the crank angle.

[0019]FIG. 1 illustrates a cylinder block 1 which comprises four cylinders 2. Each of the cylinders is assigned a pressure sensor 3 located in the region of the combustion chamber. These pressure sensors 3 are connected to inputs of a signal preparation means 5 by means of connecting lines 4. The signal preparation means 5 is part of an electronic circuit 6 which also comprises engine electronics 7. A disk 8, which, by way of example, may simultaneously form the flywheel, is arranged on a crankshaft (not shown in the drawing) of the internal combustion engine, this disk 8 being assigned an angle mark transmitter 9. This angle mark transmitter 9 is connected via a line 10 to an input of the signal preparation means 5.

[0020]FIG. 2 diagrammatically depicts the cylinder block 1 as a longitudinal section through the cylinder 2, a piston 12 being guided displaceably in the cylinder 2, the top side of the piston 12 delimiting a combustion chamber 11. At its top side, the cylinder 2 is closed off by a cylinder head 13, an intake valve 14 and an exhaust valve 17 being arranged in the cylinder head 13. The required combustion air can flow into the cylinder 2 from the induction pipe 15 through the intake valve 14, the corresponding air mass being recorded in an air mass flow meter 16. The air mass flow meter 16 is connected to the electronic circuit 6 via a line 22.

[0021] The combustion gases pass through the exhaust valve 17 into an exhaust pipe 18, which leads to a catalytic converter arrangement, which is not shown in the drawing. An exhaust-gas recirculation line 19, which branches off from the exhaust pipe 18 and opens out into the induction pipe 15 downstream of the air mass flow meter 16, is provided. In this exhaust-gas recirculation line 19 there is a quantitative recirculation sensor 20, which records the mass of exhaust gas recirculated and transmits corresponding signals via a sensor line 21 to the electronic circuit 6.

[0022] The pressure sensor 3, which has already been described in connection with FIG. 1, is arranged in the cylinder head 13 and connected to the electronic circuit 6 via the connecting line 4. Moreover, around the cylinder head 13 there is an injection valve 25, which is connected to an injection pump 23 via an injection line 26. Between the injection pump 23 and the injection valve 25 there is a measuring device 24 for measuring the fuel mass. This measuring device 24 is connected via an electric line 27 to the circuit 6, and the injection pump 23 is provided with a control line 28, the other end of which lies at the circuit 6.

[0023] The device described in FIGS. 1 and 2 makes it possible to use the pressure sensor 3 to measure the pressure profile in the combustion chamber 11. The center of gravity S of the combustion can be determined from the pressure profile, the position of the center of gravity lying at 50% of the conversion of the fuel. This relationship corresponds to the first law of thermodynamics dQ=dU+dW, i.e. the energy supplied is equal to the internal energy plus the piston work. The position of the center of gravity S changes with respect to the crank angle when the combustion profile changes, as illustrated in FIG. 3. The center of gravity S is located where 50% of the energy supplied has been converted. The dashed line in FIG. 3 illustrates that, with a changed combustion profile, for example resulting from a later start of injection, the position of the center of gravity also changes, as indicated by S₁ in FIG. 3.

[0024] The fact that the position of the center of gravity S of the combustion has direct effects on the nitrogen oxide emissions NO_(x) is clearly illustrated by FIG. 4 from which it can be seen that the NO_(x) emissions in g/kg of fuel increases as the crank angle at which the center of gravity S is reached decreases. Therefore, the result is lower NO_(x) values for later crank angles and their center of gravity S₁ or S₂.

[0025] The present invention can be used to monitor the peak pressure P_(max) and its position, based on the crank angle. Furthermore, it is possible to carry out monitoring with regard to the uniformity of combustion in the indexed cylinders. Furthermore, it is possible to use an additional NO_(x) sensor for system redundancy, in which case the measured value can be compared with the calculated value for NO_(x). The values determined for NO_(x) can be used to control and regulate exhaust-gas aftertreatment systems. The present invention is suitable not only for carrying out tests in test stands but also in particular for use in vehicles, i.e. for what is known as on-board diagnosis constant calculation and monitoring of the NO_(x) emissions is possible. 

1. A method for determining the nitrogen oxide content in oxygen-containing exhaust gases from internal combustion engines, in which, in at least one cylinder (2), a piston (12), which can move in an alternating manner therein, compresses a fuel mix in a combustion chamber (11), and in which the quantity of fuel fed to the cylinder (2) and the air mass flowing in an induction pipe (15) are recorded and are fed to an electronic circuit (6), and in which the center of gravity (S) of the combustion is determined from at least one current measured value for the engine operation, and the level of nitrogen oxide emissions is calculated from the value for the center of gravity (S) of the combustion and the values for the recorded fuel quantity and air mass.
 2. The method as claimed in claim 1, wherein the pressure profile in the combustion chamber (11) is recorded by means of a sensor (3), and at least one signal, which corresponds to the pressure profile, is fed to the electronic circuit (6), and the center of gravity (S) of the combustion is determined therefrom.
 3. The method as claimed in claim 1, wherein the electric circuit (6) comprises the engine electronics (7), in which a calculation model is stored, by means of which calculation model the center of gravity (S) of the combustion or a comparable variable, such as for example the position of the maximum energy conversion, is calculated from the current time of the start of injection (A).
 4. The method as claimed in one of claims 1 to 3, wherein the quantity of recirculated exhaust gas is recorded by means of a sensor (20), and a corresponding signal is fed to the electric circuit (6), and this signal is included in the calculation of the level of NO_(x) emissions.
 5. The method as claimed in one of claims 1 to 4, wherein the oxygen concentration is recorded, and a corresponding signal is fed to the electric circuit, and wherein this signal is included in the calculation of the level of the NO_(x) emissions.
 6. The method as claimed in one of claims 1 to 5, wherein the pressure profile in the combustion chamber (11) is recorded in each cylinder (2), and a separate calculation of the NO_(x) emissions is carried out for each cylinder (2).
 7. The method as claimed in one of claims 1 to 6, wherein an NO_(x) sensor records the NO_(x) content in the exhaust-gas stream, and a corresponding measured value is compared with the level of the calculated NO_(x) emissions.
 8. The method as claimed in one of claims 1 to 7, wherein the rotational speed of the internal combustion engine is recorded, and a corresponding signal is fed to the electric circuit, and wherein this signal is included in the calculation of the level of the NO_(x) emissions. 